Is it allowed to mix tenses in one sentence? [closed]

I have always wondered whether it is allowed to mix tenses in one sentence in english(- and in my native language, danish), which can be quite embarrassing, when the need arises!?

Examples:

Yesterday, I saw a dead cat, it is on the roof. (somehow, I know that it is still there. Is mixing allowed?)

Yesterday, I saw a dead cat, it was on the roof. (it has been removed. No problem)

Yesterday, I saw a dead cat, it ??? on the roof. (I do not know, if it is still there. What to do?)

Last week, I met my uncle, his name is Zymowch (Yes - he is still alive.)

It is certainly possible to mix tenses within a sentence.

• I feel very tired because I went to bed so late.
• I've bought a bike because I'll be without my car for a month.

The tense of the verb in each clause is chosen to convey the intended meaning.

You can fix your example sentences (which, as FumbleFingers points out, are not correct sentences) by replacing the comma with a conjunction or semi-colon. You then have:

• Yesterday I saw a dead cat on the roof and it is still there.
• Last week I met my uncle; his name is Zymowch.

which are perfectly acceptable.

I think in this case, I would use 'still'

• I saw a dead cat yesterday, and it's still on the roof.
• I saw a dead cat on the roof yesterday; it's still there.